1. Field of the Invention
The present invention relates to fiber optic gyroscopes (FOGs). More particularly, this invention pertains to a method and apparatus for regulating the operating frequency of a closed loop FOG.
2. Description of the Prior Art
German patent DE 197 53 427 C1 describes a digital phase modulator for closed loop fiber optic rate-of-rotation sensors in which the less significant portion of a binary drive signal supplied by an FOG main controller is converted into an analog signal by means of a relatively low resolution digital/analog converter to increase resolution. The analog signal is fed to a dedicated electrode on the integrated optical chip containing the digital phase modulator. Resolution can thus be increased e.g. from 8 to approximately 10 bits. The separate dedicated electrode (or, if appropriate, a separate electrode pair) is assigned directly to the digital phase modulator.
German patent application 101 30 159.6 discloses a method for avoiding bias errors due to synchronous interference in closed loop fiber-optic gyroscopes by superposing a signal on the demodulated output signal of the FOG detector. Such signal is periodic at the sampling clock rate of the FOG and is applied in the form of a modulation that is added at the digital phase modulator of a multifunctional integrated optical chip. The residue of the added modulation present in the demodulated detector signal is detected and fed to an auxiliary control loop that readjusts the operating frequency so that the added modulation tends toward zero as much as possible.
Implementation of the above method (through the use of a mixed drive signal at the phase modulator of the MIOC), which considerably increases the accuracy of FOGs, has led, in practice to difficulties. Such difficulties relate, in particular, to a conflict of objectives when it is simultaneously attempted to solve digital phase modulator resolution without increasing the structural length of the MIOC other than is described in the abovementioned German patent specification. This is particularly true when the phase modulator is intended to be operated with non-binary drive signals to increase resolution.